1. Field of the Invention
The present invention relates to an air-driven rotary contact-filter designed for use in a system for biological cleaning of waste water and more particularly to such a filter having a rotor in the shape of a spiral wound channel rotatable about a horizontal axis.
2. Description of the Prior Art
To avoid use of gear and/or other mechanical transmissions it has been proposed to provide the rotor with vanes on the periphery and to inject air under the rotor slightly off-center so that the air is caught by the vanes at one side, thus causing the rotor to rotate.
Air injected in this way apparently yields work/torque during half a rotor revolution, but in practice the torque is only present over a rather short angle of rotation, viz. in the part of the angle of rotation where the upward force is off-set most relatively to a vertical line through the center.
In order to utilize the torque resulting from this rather short distance, the rotor has to be submerged almost completely. Therefore the typical counter-pressure against injected air is at least two thirds of the rotor diameter, i.e. fans are required which have an energy waste relative to this counter-pressure.